1. Field of the Invention
The present invention relates to an anti-skid brake system for an automotive vehicle.
2. Description of the Related Art
Recently, an increasing number of automotive vehicles are loaded with an anti-skid brake system (usually called "ABS system") which is arranged to prevent the wheel from locking at the moment of braking. In order to operate the anti-skid brake system, i.e. to control the braking force, there may be three modes for controlling the braking force, a first mode being to reduce the braking force, a second mode being to elevate the braking force, and a third mode being to hold the braking force.
It is noted herein that the determination if the wheel is locked is made on the basis of a slip value given from an assumed vehicle speed to be determined theoretically and an actual wheel speed. Hence, the determination if the control of the braking force should be started can be implemented by determining if the slip value exceeds a predetermined threshold value.
Further, the magnitude of deceleration can be determined with ease and with accuracy by differentiating the wheel speeds detected by wheel speed sensors, and the magnitude of deceleration indicates a behavior of the wheels with considerable accuracy. Hence, if the ABS control should be started can be determined by determining if the magnitude of deceleration of the wheel exceeds a predetermined threshold value. Japanese Patent Laid-open Publication (kokoku) No. 61-24,220 discloses an example that takes advantage of the magnitude of deceleration of the wheel for the ABS control. This publication proposes the determination if the control of reducing the braking force has been finished by determining if the magnitude of deceleration of the wheel exceeds a predetermined threshold value.
On the other hand, strong demands in the ABS control have recently been made to shorten a distance required upon braking the wheels, i.e. a braking distance. In order to shorten the braking distance, it is disadvantageous to start the ABS control at an unnecessarily early timing. However, the ABS control is likely to be started at an unnecessarily early timing when the determination of starting the ABS control is made by taking advantage of the slip value or the magnitude of deceleration of the wheel. More specifically, particularly when the wheels are braked rapidly and strongly, acoustic tires serving as members for contacting the wheels with a road surface are forcibly transformed due to elasticity in the initial stage of braking and then returned to their original shapes due to elasticity. This appears as a phenomenon in which the magnitude of deceleration of the wheel becomes larger once in the initial stage of braking and then smaller. In other words, the slip value becomes once smaller and then larger. Hence, if the ABS control starts on the basis of the magnitude of deceleration of the wheel or the slip value, which appears in the initial stage of the phenomenon, there is the risk that the ABS control is started at an unnecessarily early timing.